A master cylinder is a device for generating hydraulic power in a hydraulic brake system.
A master cylinder includes a cylinder housing and a piston which is movably disposed in the cylinder housing. Further, a sealing member which is installed in an inner surface of the housing, and the piston is provided with a communication hole. The sealing member has a cup shape and is also called a cup seal.
The cylinder housing and the piston form cooperatively a pressure chamber for generating hydraulic pressure at a side toward which the piston moves forward, and a hydraulic supplying chamber which communicates with a reservoir is formed between the cylinder housing and an outer surface of the piston. The sealing member partitions the hydraulic supplying chamber and the pressure chamber.
In this configuration, a shape of the sealing member is changed during the movement of the piston, and if the changed shape of the sealing member does not return to its original shape during the movement of the piston, an oil passage may be blocked and thus there may be problems of meaningless backward movement and of residual pressure.